The use of material of natural origin in the preparation of labeling adhesives has been widely described and industrially exploited for decades. That is in particular the case for aqueous glues based on casein and/or on starchy materials intended for affixing paper labels to glass supports.
In general, a labeling adhesive should satisfy all the criteria a) to i) below:
a) possess general rheological properties suitable for the unit operations preceding the actual process of sticking between the label and the support and in particular suitable for the increasingly fast and efficient processes and devices for the preparation, transport and deposition of the glues, for positioning the labels on their supports and for bottling.
It should be possible to obtain these suitable rheological properties for adhesives having relatively high dry solids contents, i.e. greater than 40% while having the desired viscosities, for example of between 20,000 and 150,000 mPa·s (at 25° C.) for glues intended for labeling glass bottles.
b) possess in particular sufficient “tack” properties, i.e. a sufficient capacity, before any significant loss of water from the adhesive initiating the actual bonding phenomenon, to keep assembled the two substrates (label and support) as soon as they are placed one onto the other.
This minimum tack should for example make it possible to avoid a paper label which has just been affixed to a glass bottle sliding along the latter. This result must be obtained on a bottle whose surface is either wet or dry.
c) possess in particular a so-called “short” texture, i.e. a texture allowing effective and clean distribution of the glue, including at high speeds, and in particular limiting the formation of “filaments”, smears and/or discharges, which are sources of dirt marks on the labeling machine.
d) develop, after positioning the label on its support, and then preserve sufficient adhesive properties, this being in all types of environment. The adhesive should in particular possess good resistance to water, including condensation water. Bottles (of wine, champagne, beer and the like) are often placed in a refrigerator, an ice bucket or in cold water in order to cool their content. It is then essential, including from an esthetic point of view, that the label does not become detached from its support in such an environment.
e) allow, in view of increasingly frequent operations for reusing or recycling glass or plastic articles, the detachment of the label under simple and inexpensive conditions, without leaving marks or residues on said articles. The adhesive should in particular be easily and completely removable from the surface of the article by a conventional treatment of washing at high temperature, optionally in an alkaline bath.
f) possess acceptable organoleptic characteristics (including transparency, color, odor) for the manufacturer and then the applicator (bottler for example) of the adhesive and for the successive handlers or users of the marketed labeled article.
g) preserve during storage, if possible for at least several months, the general or specific rheological characteristics, the adhesive characteristics, the possibilities of removal and the organoleptic characteristics as mentioned in points a) to f) above.
h) exhibit the harmlessness and good biodegradability, in particular in relation to ever restrictive regulations in terms of protection of humans and of their environment.
i) have the simplest and most “natural” composition possible and the lowest cost price possible.
It appears however that both casein and starchy materials do not always make it possible to satisfy all the abovementioned criteria a) to i).
Casein is a protein of animal origin (protein extracted from milk) which is soluble in a basic medium and which has the property of very considerably viscosifying upon cooling. This makes this polymer particularly of interest in labeling operations on cold and wet glass. Because of the thermoreversibility of this phenomenon, casein is additionally relatively easily removable by washing at high temperature.
However, casein has the following disadvantages:                poor suitability for labeling operations on hot glass (pasteurized bottles for example) precisely because of the high dependence of its viscosity on the temperature conditions,        need for washing in a basic medium for the purpose of operations for recycling bottles and other articles,        strong characteristic odor and pronounced coloration,        high cost, in particular compared with that of starchy materials, and subject to large fluctuations.        
Starchy materials have overall the advantages of a relatively low and stable cost, ease of removal with water and appropriate organoleptic properties.
Among these starchy materials, the so-called “amylopectin-rich” or waxy starches confer most particularly on the glues into which they are introduced, a very good storage stability and, through chemical modification, a “short” texture as defined above. That is, for example, the case for the product called “DEXYLOSE® I 231”, developed and marketed by the applicant.
However, in general, glues based on starchy materials have the following disadvantages:                lack of tack on a cold and/or wet support, and        poor resistance to water.        
The abovementioned polymers (casein, starchy materials) therefore do not always make it possible to have adhesives which satisfy all the abovementioned criteria a) to i).
One of the problems linked to the use of these polymers in labeling adhesives consists in particular in simultaneously obtaining characteristics of good resistance to water (cf. abovementioned criterion d)) and of easy removal by simple washing with hot water (cf. abovementioned criterion f)), it being possible to consider these characteristics as being antinomic.
For the purposes of obtaining an acceptable compromise between these characteristics, aqueous adhesives have been proposed, which combine starches and synthetic resins such as ammonium salts of styrene-maleic anhydride copolymers. Such resins, which are not very biodegradable, cause environmental problems, in particular in the water for washing recycled bottles. In addition, as underlined in U.S. Pat. No. 4,336,166, these additives do not exhibit the desired rheological and stability characteristics. To this effect, said patent proposes replacing said ammonium salts with zinc salts of the same styrene-maleic anhydride resins.
However, the aqueous compositions recommended in this patent U.S. Pat. No. 4,336,166 are in practice:                relatively complex because they also contain, in particular, bentonite and a plasticizer system based on glycol benzoates,        relatively expensive and not very biodegradable because of the relatively high proportion (3 to 20% dry/dry) as resin emulsion and the relatively low contentin starch(es) (35% dry/dry maximum)        
Other solutions have been proposed in order to improve the rheological and stability characteristics of aqueous glues intended for labeling glass articles.
U.S. Pat. No. 4,462,838 describes polymeric compositions necessarily containing an oxidized starch ether, generally a hydroxypropylated ether of oxidized potato starch, and at least a second polymer of natural origin (other starch derivatives, casein and/or cellulose derivatives) or of synthetic origin (polyvinyl alcohol, polyacrylamide, polyvinyl pyrrolidone), the second polymer necessarily having to be soluble in water and to be used before the oxidized starch ether during the preparation of the adhesive.
The adhesive compositions exemplified in this document are complex and expensive because they use, in the majority of cases, at least three different polymers out of which two are modified starches, namely, in a precise order, a cationic starch solubilized in water and then oxidized starch ether necessarily present.
Moreover, the characteristics of resistance to water and the possibilities of removal with hot water of these compositions are not studied in this patent U.S. Pat. No. 4,462,838.
For its part, U.S. Pat. No. 4,675,351 recommends a combination between solubilized soybean protein(s), synthetic polymer and starch or flour, in order to obtain an aqueous labeling glue having appropriate rheological characteristics, the water resistance of said glue being conferred by the use of zinc oxide.
The adhesives obtained are presented as being easily removable by washing with hot alkaline water.
However, no result is presented as to the possible preservation of all of these characteristics after the storage of such glues.
Moreover, the method for producing these adhesives appears to be relatively complex and expensive. In any case, it necessarily involves the significant use (of 1 to 10% by weight) of a synthetic polymer (polyvinyl acetate emulsion) and a limited use of starches (15% by weight maximum).
U.S. Pat. No. 4,804,414 recommends the use of a resin consisting of the solubilized product of neutralization of an alkaline base and a rosin composition selected so as to confer good characteristics of resistance to water and removal in alkaline water on these aqueous starchy glues. It appears, however, that in practice, in order to obtain such characteristics, the starch must necessarily a) be a so-called “amylopectin-rich” or waxy starch, i.e. containing more than 5% by weight of amylose and b) be combined with a high proportion (40 to 50% weight of starch) of selected rosin resin.
More recently, selected resins or gums based on rosin have also been combined with casein and possibly with a starch, in the preparation of aqueous labeling glues as described in U.S. Pat. Nos. 5,455,066 and 5,441,562. But here again, said resins or gums represent, according to the examples of said patents, a very high percentage (at least 30-35% by weight) of the dry solids content of said glues. In addition, to obtain good, in particular rheological and/or adhesive, properties, said glues must contain substantial levels of mono- or polyhydric alcohols such as methanol, ethylene glycol or glycerol.
U.S. Pat. No. 5,641,349 describes, in its EXAMPLE 31, an aqueous labeling glue for bottles based on cassava starch treated according to a particular so-called thermal inhibition process.
This adhesive, which is not very biodegradable, contains a high proportion, namely 50%, expressed by weight relative to the weight of cassava starch, of a synthetic resin of the ethylene-vinyl acetate (EVA resin) type.
The applicative performance features of the glue obtained, in particular in terms of resistance to water and removal with hot (alkaline) water, are not at all detailed.
More recently still, the use of EVA resins has been recommended and exemplified:                in combination in particular with zinc oxide, in the context of the preparation of aqueous labeling glues of improved thixotropy based on casein as described in application WO 01/08984, or        in combination in particular with ammonium hydroxide and rosin resin, in the context of the preparation of aqueous labeling glues based on casein or a starchy mixture (starch blend of unspecified composition) having good resistance to cold water as described in application WO 01/85866.        
Finally, it has been proposed, in application US 2003/0064178, or in the equivalent application WO 03/029378, to prepare aqueous labeling glues especially suitable for the positioning of transparent plastic labels on containers made of glass and characteristically containing gelatin and at least one starch.
It is stated, in very broad terms, that said starch may be derived from cereals, tubers, roots, legumes or fruits, the term “pea” being mentioned within a very long list of native starches which can be potentially used.
However, very preferably, these glues contain an “amylopectin-rich” or waxy starch, it being possible for the latter in particular to consist of a pregelatinized modified waxy maize starch such as the abovementioned product “DEXYLOSE® I 231”, which is used in all the examples in this document.
Some teachings of this application US 2003/0064178 or WO 03/029378 are in addition ambiguous and contradictory in the sense that:                on the one hand, it is stated that the “amylopectin-rich” starch may be advantageously combined with an “amylose-rich” starch, but        on the other hand, the only starches cited or exemplified in combination with said DEXYLOSE® I 231 are respectively a) the product called “COLLYS® BR” marketed by the applicant and consisting of a crosslinked wheat starch or b) the product called “SOLVICOL GP 45” marketed by the company AVEBE and presented as a hydrolyzed potato starch.        
It has to be recognized that neither of these two products is an “amylose-rich” starch since the first is derived from wheat starch and therefore has an amylose content of the order of 24 to 28% and the second is derived from potato starch and therefore has an amylose content of the order of 20 to 23%.
None of said examples therefore describes the use of a truly “amylose-rich” starch. Moreover, all the compositions exemplified contain, inter alia, a crosslinking agent consisting of zinc carbonate. In addition, it is simply mentioned in general terms in this application that the adhesives prepared have a good wet tack and good characteristics of clarity and transparency. No detail or figure is given in particular as regards the characteristics of resistance to water, the possibilities of removal with hot water and the characteristics of storage stability of these adhesives or of their applicability for other labels than those made of plastic, in particular for paper labels.
As a result of the foregoing, no means currently exists which makes it possible to provide a labeling adhesive, applicable in particular to paper-based labels, which satisfies all the abovementioned criteria a) to i).
In particular, no means exists in this field which makes it possible in a simple, inexpensive and ecologically acceptable manner, to obtain good compromise:
a) on the one hand, between a good resistance to water (cf. abovementioned criterion d)) and possibilities for easy removal by simply washing with hot water (cf. abovementioned criterion e)) and a high biodegradability (cf. abovementioned criterion h)),
b) on the other hand, between a good resistance to water, good rheological properties (cf. abovementioned criteria a) to c)) and a good storage stability (cf. abovementioned criterion g)).
It is to the applicant company's credit to have found that such a means could consist in the use of a selected starch, in the present case, from a legume starch, the latter additionally having a particular amylose content.